The present invention relates to a cathode ray tube, and in particular to a color cathode ray tube having an electron gun employing an internal voltage-dividing resistor.
Color cathode ray tubes used in TV receivers or information terminals, house an electron gun for emitting a plurality (usually three) of electron beams at one end of an evacuated envelope, a phosphor screen formed of phosphors coated on an inner surface of the evacuated envelope at the other end thereof for emitting light of a plurality (usually three) of colors, and a shadow mask which is closely spaced from the phosphor screen and serves as a color selection electrode. The electron beams emitted from the electron gun are deflected to scan the phosphor screen horizontally and vertically to form a rectangular raster by magnetic fields generated by a deflection yoke mounted externally of the evacuated envelope and display a desired image on the phosphor screen.
FIG. 8 is a cross-sectional view for explaining an exemplary configuration of a color cathode ray tube, and in FIG. 8, reference numeral 1 denotes a panel portion, 2 is a neck portion for housing an in-line type electron gun 9, 3 is a funnel portion for connecting the panel portion 1 and the neck portion 2, 4 is a phosphor screen, 5 is a shadow mask, 6 is a mask frame, 7 is a magnetic shield, 8 is a mask suspension mechanism, 10 is a deflection yoke, 11 is an internal conductive coating, 12 is a shield cup, 13 is a contact spring, 14 is a getter and 15 are stem pins.
In this color cathode ray tube, an evacuated envelope is formed by the panel portion 1, the neck portion 2 and the funnel portion 3, and electron beams 16 emitted from the electron gun 9 housed in the neck portion 2 scan the phosphor screen 4 two-dimensionally by being subjected to the horizontal and vertical deflection magnetic fields produced by the deflection yoke 10.
The electron beams 16 are modulated in amount by video signals supplied via the stem pins 15, are color-selected by the shadow mask 5 disposed immediately in front of the phosphor screen 4, and impinge upon the phosphors of the corresponding primary colors to reproduce a desired color image.
Such cathode ray tubes employ a multistage focus lens system to obtain sufficiently small electron beam spots over the entire phosphor screen.
Japanese Patent Application Laid-open No. Hei 10-255682 (laid-open on Sep. 25, 1998), for example, discloses an xe2x80x9cextended field lensxe2x80x9d serving as a main lens formed by disposing an intermediate electrode between an anode and a focus electrode. FIG. 9 is a schematic longitudinal cross-sectional view of an electron gun of a cathode ray tube disclosed in Japanese Patent Application Laid-open No. Hei 10-255682 and FIG. 10 is a cross-sectional view taken along line Xxe2x80x94X of the electron gun shown in FIG. 9. The electron gun is of the extended field lens type comprising three equally spaced coplanar cathodes 309 (one for each electron beam), a first electrode 301, a second electrode 302, a third electrode 303, a fourth electrode 304, a 5-1st electrode (a focus electrode) 305, a 5-2nd electrode (a focus electrode) 306, an intermediate electrode 310, a sixth electrode (an anode electrode) 307 and a shield cup 308 arranged coaxially in the order named from the cathodes 309, and the cathodes and the electrodes are fixed in predetermined spaced relationship on a pair of glass beads 311.
A voltage-dividing resistor 312 fabricated on a ceramic substrate is housed within the cathode ray tube to obtain a voltage to be supplied to the intermediate electrode 310 within the cathode ray tube, and the voltage-dividing resistor 312 is fixed to one of the glass beads 311. A metal wire 314a surrounds the glass beads 311 and the voltage-dividing resistor 312 and is welded to the intermediate electrode 310 as shown in FIG. 10.
The electrons emitted from the cathodes 309 are focused by a prefocus lens formed by the cathodes 309, the first electrode 301, the second electrode 302 and the third electrode 303, next by a pre-main lens formed by the third electrode 303, the fourth electrode 304 and the 5-1st electrode 305, and then by a main lens formed by the 5-2nd electrode 306, the intermediate electrode 310 and the sixth electrode 307, onto a phosphor screen, and form an image on the viewing screen of the cathode ray tube.
The voltage applied to the intermediate electrode 310 is selected lower than anode voltage, but higher than voltages applied to the focus electrodes by dividing the anode voltage using the voltage-dividing resistor 312. Provision of the intermediate electrode 310 forms a lens of the extended field type in which the potential distribution along the tube axis is made gentle from the anode electrode to the focus electrodes, reduces spherical aberration and consequently the diameter of the electron beam spots is reduced.
As shown in FIG. 10, the amount of electrical charges accumulated on the inner wall of a neck glass 317 is stabilized by attaching the metal wire 314a to the intermediate electrode 310 such that the metal wire 314a surrounds the glass bead 311 and the voltage-dividing resistor 312.
After the completed electron gun is inserted into the neck glass 317, a portion of metal contained in the metal wire 314a is evaporated to form metal films (not shown) on the inner wall of the neck glass 317 and the surface of the voltage-dividing resistor 312 and the glass bead 311 by heating the metal wire 314a using an external radio-frequency induction heater such that more stable potential is established on the inner wall of the neck glass 317.
In the manufacture of a cathode ray tube, after the cathode ray tube has been exhausted of gases and sealed, so-called spot-knocking (high-voltage stabilization) of applying a high voltage of about twice the normal operating voltage for the cathode ray tube to its anode is carried out to remove projections in electrodes of the electron gun or foreign particles within the cathode ray tube by forcing arcing between the electrodes and between the electrodes and the inner wall of the neck portion and to thereby prevent occurrence of arcing within the cathode ray tube during the normal operation of the completed cathode ray tube.
But, in a cathode ray tube employing the extended field lens formed by applying a voltage divided from the anode voltage using an internal voltage-dividing resistor to the intermediate electrode and the above-mentioned metal wire for suppression of discharge attached to and facing a focus electrode upstream of the intermediate electrode, when the spot-knocking of applying a high voltage of about 60 kv, for example, to the anode electrode is carried out with all the electrodes except for the anode electrode and the intermediate electrode being grounded, cracking occurs passing through the layers of the internal voltage-dividing resistor because the metal wire for suppression of discharge surrounding the voltage-dividing resistor is grounded and therefore a voltage difference of about 30 kV is produced between the metal wire and the resistance element of the voltage-dividing resistor, and consequently, there has been a problem in that voltages sufficiently high for spot-knocking are not generated between the anode electrode and the intermediate electrode adjacent thereto and between the intermediate electrode and another electrode facing the cathode side of the intermediate electrode, as a result sufficient effects of spot-knocking are not obtained and satisfactory withstand voltage characteristics are not secured within the cathode ray tube.
It is an object of the present invention to provide a cathode ray tube incorporating an internal voltage-dividing resistor and having withstand voltage characteristics improved by heightening effects of spot-knocking preventing occurrence of cracking passing through the layers of the internal voltage-dividing resistor during the spot-knocking procedure.
To accomplish the above object, in accordance with an embodiment of the present invention, there is provided a cathode ray tube comprising: an evacuated envelope comprising a panel portion having a phosphor screen formed on an inner surface thereof, a neck portion and a funnel portion connecting said panel portion and said neck portion; an electron gun housed in said neck portion comprising at least one cathode, a first grid electrode, a second grid electrode, a plurality of focus electrodes and an anode arranged in the order named for focusing at least one electron beam emitted from said at least one cathode on said phosphor screen, said at least one cathode, said first grid electrode, said second grid electrode, said plurality of focus electrodes and said anode being fixed in predetermined axially spaced relationship by at least two glass beads; a voltage-dividing resistor attached to one of said at least two glass beads for producing an intermediate voltage to be applied to a first one of said plurality of focus electrodes adjacent to said anode by dividing a voltage applied to said anode; and a metal conductor facing and attached to a second one of said plurality of focus electrodes to surround said voltage-dividing resistor and said one of said at least two glass beads, said second one of said plurality of focus electrodes being disposed upstream of said first one of said plurality of focus electrodes; said voltage-dividing resistor comprising a first overcoat insulating film, a resistance element, an insulating substrate and a second overcoat insulating film stacked in the order named from said first overcoat insulating film facing said one of said at least two glass beads, and a portion of said second overcoat insulating film containing a region thereof facing said metal conductor being made locally thicker than a remainder of said second overcoat insulating film.